This invention involves the use of intense magnetic fields to accelerate the chemical changes associated with the formation of graphitic nanotubes. This invention pertains to a new process and apparatus for the massive synthesis of carbon nanotubes using strong external magnetic fields to enhance the formation and growth by reinforcing the intrinsic magnetic mechanisms occurring during the formation.
The carbon nanotube is a hollow tube with walls made of graphene and a capping dome containing C6 conjugated aromatic rings and a few C5 member rings to alleviate curvaceous strain on the bonds. Such macromolecules of graphene are known as fullerenes. The carbon nanotube is a fullerene of great importance.
The mass production of carbon nanotubes for diverse applications is currently among the most challenging problems in nanochemistry, potentially revolutionizing many important areas of material science, particularly nanoelectronics and structural materials. The importance of enhanced formation and mass production follows from the rich potential for applications.
For example, the potential applications in nanoelectronics result from the atomic level perfection. Future nanoelectronic materials will require atomic level perfection for optimum device performance. Such a demand is unrealistic for most materials. However, the nanotube is an exception to this impracticality. The nanotubes exhibit intrinsic perfection due to the chemically bonded network, where thermodynamics restricts defect formation. Such perfection in the carbon nanotube allows ideal exploitation of mesoscopic and molecular properties, where atomic defects are intolerable. The perfection allows nice thermal and electrical conductivity in the direction of the tube axis due to the extended electronic resonance. The chemically bonded network also leads to the maximum possible strength among materials. The strength of nanotubes will contribute to many future structural applications. This optimum strength reflects the extremely stable, doubly covalently bonded network. Carbon nanotubes therefore exhibit the largest strength/mass ratio known for fibers. This threshold for maximum strength requires the existence of atomic perfection in these nanotubes.
Because of these and other possibilities, the large scale production of carbon nanotubes is currently being investigated for applications in nano-computers, strong structural support, catalyst support, scan tunneling microscopy tips and hydrogen storage in order to make use of these unique properties. Unfortunately, the large-scale preparation of the carbon nanotubes has been a challenge with no breakthroughs, except for the new approach described here. With all the prospective applications, large-scale production will be necessary to realize these many new applications. First, the older techniques are considered along with problems associated with these techniques. Then the new invention is described with detailed explanations as to how the new art resolves difficulties of the older art.
The carbon nanotube, C60, and other fullerenes were initially discovered in the debris of DC arc discharges. Therefore, the earlier preparations of the nanotubes used DC arc discharges with the graphite and the catalytic metal buried within the anode. In this process, the anode is packed with the metal catalyst and a carbon source. The ablation during an intense pulse of electric current produces high temperature and generates plasma, which contains ions and atoms of the eroded anode material. These ions and atoms nucleate the fullerenes. Later strategies used modified arc discharge, which synchronously arched and helium quenched plasma in an effort to increase the formation of graphite encapsulated metal particles and the formation of carbon nanotubes. Laser ablation of the anode was the next effort used to increase production.
In recent years, with the discovery of catalytic chemical vapor deposition (CCVD), the emphasis for mass production has shifted from arc discharge to CCVD techniques. CCVD involves the formation of carbon nanotubes by the catalytic decomposition of hydrocarbons on the surface of Fe, Co, and Ni nanoparticles. The CCVD technique is a continuous process, whereas the arc discharge process is a batch process. The continuous CCVD process is an ideal approach for the industrial synthesis of nanotubes, since CVD allows more strict control of processing conditions and CVD has demonstrated itself to be a useful method for industrial production of many other materials. Although the CCVD effort has increased the production rate, current CCVD methods still do not achieve the desired bulky formation rates.
Unfortunately, past attempts to increase growth rates of nanotubes by CVD have failed due to the poisoning of the catalytic nanoparticles. These attempts involved the older upscaling methods. The older upscaling methods involved changing reactor parameters such as increasing concentration, increasing temperature, and changing catalyst content so as to increase growth rate. These efforts have not been very fruitful for achieving commercial growth rates. Higher hydrocarbon concentrations cause poisoning of the catalyst. Poisoning is the loss of catalytic activity due to carbiding. Carbiding is the formation of metal-carbon chemical bonds on and within the catalytic metal nanoparticle. Supersaturation is the state of the metal nanocatalyst in which the carbon concentration exceeds the equilibrium concentration for carbon precipitation. High concentrations also contribute to multi-walled carbon nanotubes and the accumulation of amorphous carbon along the tube backbone. High growth temperature leads to impractical production cost. The adulteration of the catalyst can introduce impurities into the nanotube and broaden the size distribution. These problems of traditional upscaling arise because the catalytic mechanism of carbon nanotube growth involves novel phenomena alien to the older art.
The following synopsis provides a condensed overall disclosure of references on many of these difficulties associated with trying to apply the traditional art to the carbon nanotube mass production. The current state of understanding of the nanotube formation in terms of the older chemical mechanisms and the history behind developing strategies for addressing these difficulties are disclosed. The current level of technology for nanotube formation and growth is disclosed. Finally, the new revelation from this invention for mass production is presented. The new revelation is considered in details, giving a detailed account of: 1) the new process in comparison to older processes, 2) the new mechanism based upon the intrinsic magnetic field and the consequences of superposing and reinforcing by an external magnetic field and 3) the new design and apparatus. But, first the references are given below:
R. T. K. Baker et al., xe2x80x9cNucleation and Growth of Carbon Deposits from the Nickel Catalyzed Decomposition of Acetylene,xe2x80x9d Journal of Catalysis vol. 266, pp. 51-62 (1972) discloses the growth of graphite laminae from nickel particles (300 nm diameter) by the catalytic decomposition of acetylene at 1300K. Furthermore, they disclose terminated growth after 15 seconds at 870K upon the accumulation of amorphous carbon about the catalyst. The regeneration is reported after adding H2 at 1100K or O2 at 1000K. The filament formation is accounted for by acetylene adsorption and decomposition at one facet of the catalyst with subsequent internal carbon diffusion to and precipitation at an opposite facet of the particle. The diffusion process is disclosed as temperature and concentration driven.
J. C. Shelton et al., xe2x80x9cEquilibrium Segregation of Carbon to Nickel(111) Surface: A Surface Phase Transition,xe2x80x9d Surface Science vol. 43, pp. 493-520 (1974) discloses the precipitation on the (111) surface of Ni of at least three carbonaceous states: a high temperature dilute carbon phase, a condensed graphitic monolayer and a multilayered epitaxial graphite precipitate.
S. E. Stein and A. Fahr, xe2x80x9cHigh Temperature Stabilities of Hydrocarbon,xe2x80x9d Journal of Physical Chemistry vol. 89, pp. 3714-3725 (1985) discloses the thermodynamic stability of polyaromatic species relative to saturated hydrocarbons at 1500-3000K and further discloses possible pathways for hydrocarbon decomposition to polyaromatic species.
G. G. Tibbets et al., xe2x80x9cAn Adsorption-Diffusion Isotherm and Its Application to the Growth of Carbon Filaments on Iron Catalyst Particles,xe2x80x9d Carbon vol. 25, pp. 367-375 (1987) discloses that the concentration gradient drives the carbon diffusion through the catalyst particle. They further disclose the poisoning by the collision of the catalyst particle with a viscous hydrocarbon particle.
R. T. K. Baker et al., xe2x80x9cCatalytic Growth of Carbon Filaments,xe2x80x9d Carbon vol. 27, pp. 315-323() discloses that the adulteration of the catalyst with Cu leads to an increased rate of nanotube formation with branch nanotube nucleation and growth on a single catalyst particle. The symmetrical branching suggests the carbon filament develop from specific crystallographic planes of the catalyst particle.
Haddon et al., xe2x80x9cExperimental and Theoretical Determination of the Magnetic Susceptibility of C60 and C70,xe2x80x9d Nature vol. 350, pp. 46(1991) discloses the differing diamagnetic susceptibilities of C60, C70 and graphite. They disclose that the difference results from the 5 member paramagnetic rings, which counter the ring current of the diamagnetic C6 member ring. This opposing paramagnetic effect of the C5 member ring causes the following relative diamagnetic susceptibility: Graphite greater than C70 greater than C60. This decrease reflects the increasing ratio of 5 to 6 member rings.
Schnabel et al., xe2x80x9cEvidence of Low Pressure Catalysis in the Gas Phase by a Naked Metal Cluster: The Growth of Benzene Precursors on Fe4+,xe2x80x9d Journal of Physical Chemistry vol. 95, pp. 9688-9694(1991) discloses the catalytic capacity for Fe4+ to form benzene from ethylene and cyclopropane. Hence, the polynuclear aromatics can be formed from hydrocarbons at lower decomposition temperature due to catalysis.
Pan et al., xe2x80x9cReaction of Co1-4+ and Co4(CO)n with Cyclohexane: Cxe2x80x94H Activation as a Function of Cluster Size and Ligand Substitution,xe2x80x9d Journal of the American Chemical Society vol. 113, pp. 2406-2411(1991) discloses that the catalytic capacity of Co clusters depends on the cluster size, which reflects the nature of the frontier orbitals. They disclose the increased reactivity of the 3d transition metal clusters as being due to the lower spin in the frontier orbitals, which results in the greater influence of s orbitals. For second and third series transition metals, d orbitals play a stronger role. The accepting ability of the S orbital of the catalyst leads to the conversion of saturated hydrocarbons to polycylic aromatic hydrocarbon (PAH).
Endo and Koto, xe2x80x9cFormation of Carbon Nanofibers,xe2x80x9d Journal of Physical Chemistry,xe2x80x9d vol. 96, pp. 6941-6944(1992) discloses the existence of giant fullerenes and the growth by the insertion of sp2 carbon in the arc discharge process.
Saito et al., xe2x80x9cTopological Defects in Large Fullerenes,xe2x80x9d Chemical Physics Letters, vol. 195, pp. 537-541(1992) disclose the efficient bond rearranging reactions wherein the motions of five member and seven member ring pairs are demonstrated with subsequent annihilation by collision with a second five member ring.
Rodriquez et al., xe2x80x9cA Review of Catalytically Grown Carbon Nanofibers,xe2x80x9d Journal of Materials Res. Vol. 8, pp. 3233-3250(1993) discloses the preferential decomposition and graphitization on certain facets of the catalyst, discloses the hydrogen and sulfur promotion of catalysis, discloses the absence of carbide and discloses the importance of wetting of metal by graphite precipitate.
Ivanov et al., xe2x80x9cThe Study of Carbon Nanotubules Produced by Catalytic Methods,xe2x80x9d Chemical Physics Letters vol. 223, pp. 329-335 (1994) discloses the catalytic chemical vapor deposition of carbon nanotubes with Co as the best catalyst among Co, Fe and Ni catalysts.
Rose and Shore, xe2x80x9cElastic Constants of the Transition Metals from a Uniform Electron Gases,xe2x80x9d Physical Review B vol. 49 (17) pp. 11566(1994) discloses the trends in bond strength among the transition metals, demonstrating experimentally and computationally the unusual behavior of the ferro-metals.
Guerret-Piecourt et al., xe2x80x9cRelation between Metal Electronic Structure and Morphology of Metal Compounds Inside Carbon Nanotubes,xe2x80x9d Nature vol. 372, pp.761-764 (1994) discloses that the ability to encapsulate transition metals within carbon nanotubes depends on the incompleteness of the electronic shell in the most stable ionic state of the metal.
I. M. Billas et al., xe2x80x9cMagnetism from the Atom to the Bulk in Iron, Cobalt and Nickel Clusters,xe2x80x9d Science vol. 265, pp. 1682-1684(1994) discloses the atomic like magnetic moments of small Fe, Co and Ni clusters containing less than 30 atoms. Moreover, they disclose the oscillating moments in these nanoparticles due to surface induced spin density waves (Friedel waves). Furthermore, Fe clusters exhibit a temperature dependent phase transition between a high moment BCC phase and a low moment FCC phase.
H. Hjiki and T. Ando, xe2x80x9cEnergy Bands of Carbon Nanotubes in Magnetic Fields,xe2x80x9d Journal of Physical Society of Japan vol. 65, pp. 505(1995) discloses the splitting of electronic bands of carbon by a magnetic field normal to the tube axis.
Wang et al., xe2x80x9cCarbon Nanotube Synthesized in a Hydrogen Arc Discharge,xe2x80x9d Applied Physics Letters vol. 66, pp. 2430-2432(1995) discloses the formation of opened ended carbon nanotubes in the presence of H2.
Setlur et al., xe2x80x9cA Method for Synthesizing Large Quantities of Carbon Nanotubes and Encapsulated Copper Nanowires,xe2x80x9d Applied Physics Letters, vol. 63, pp. 345-347(1996) discloses the role of H2 in generating polycylic aromatic hydrocarbons during arc discharge and the ability of Cu to graphitize polycylic aromatic hydrocarbons (PAH).
Jia et al., xe2x80x9cPreparation and Properties of Ferromagnetic Carbon-Coated Fe, Co and Ni Nanoparticles,xe2x80x9d Journal of Applied Physics, vol. 80, pp. 103-108 (1996) discloses the interaction between Fe, Co and Ni catalysts and the graphite precipitate with graphitization occurring at 600xc2x0 C. for Fe and Co. But graphitization occurs at 900xc2x0 C. for Ni (in comparison to 3000K for xc2x0C.).
Chambers et al., xe2x80x9cInfluence of Copper on the Structural Characteristics of Carbon Nanofibers Produced from the Cobalt-Catalyzed Decomposition of Ethylene,xe2x80x9d Journal of Materials Res. Vol.11 pp. 430-437 (1996) discloses the ability of Cu in a Co/Cu alloy to enhance the growth rate of smaller nanotubes relative to the rate formed by pure Co. The enhanced formation of the smaller tubes is disclosed as resulting from the facilitation of surface reconstruction by carbide formation allowing multi-faceted particle surface morphology leading to bimodal growth and smaller nanotubes. Cu prevents graphitic overlayer and catalyst poisoning.
Colbert et al., xe2x80x9cGrowth and Sintering of Fullerenes Nanotubes,xe2x80x9d Science vol. 266, pp. 1218-1222(1995) discloses a mechanism for nanotube formation in arc discharge where in the absence of catalyst the tube opening is stabilized by the strong electric field of the plasma.
Seraphin et al., xe2x80x9cFilling the Carbon Nanocage,xe2x80x9d Journal of Applied Physics vol.80, pp. 2097-2104(1996) discloses the propensity for graphitic encapsulation of transition metals depends on the existence of its carbide, interfacial interaction and stability with the graphite shell and the processing conditions: 1) weak carbides are encapsulated as carbides, 2) unstable carbides are not encapsulated but form a graphitic cage, 3) stable carbides are not encapsulated, and 4) metastable carbides are not encapsulated but form nanotubes.
Host et al., xe2x80x9cGraphite Encapsulated Nanocrystals Produced Using a Low Carbon: Metal Ratio,xe2x80x9d Journal of Material Res. Vol. 12, pp. 1268-1273(1997) discloses the ability to encapsulate rather than form nanotubes by using high metal/carbon concentrations. The effect of particle size on the structure (high temperature fcc or low temperature phase) is also disclosed.
Kanzow et al., xe2x80x9cLaser Assisted Production of Multi-Walled Carbon Nanotubes from Acetylene,xe2x80x9d Chemical Physics Letters vol. 295, pp. 525-529 (1998) discloses the effect of freezing point depression by carbon on the catalyst and the effect of particle size on nanotube formation (smaller favor) to graphitic encapsulation (larger favor).
Kong et al., xe2x80x9cChemical Vapor Deposition of Methane for Single Walled Carbon Nanotubes,xe2x80x9d Chemical Physics Letters vol. 292, pp. 567-574(1998) discloses the importance of carbon source (CH4) to favoring single over multi walled nanotubes. This result is related to the effect low carbon concentration on increasing single wall nanotube formation.
Kanzow et al., xe2x80x9cFormation Mechanism of Single Walled Carbon Nanotubes on Liquid-Metal Particles,xe2x80x9d Physical Review B vol. 60, pp. 11180-11186(1999) discloses a nucleation model for carbon nanotube formation wherein the nanotubes nucleate from graphitic sheets parallel to the catalyst. The sheets undergo bending at the high synthesis temperature by coordinated vibrations to form domes, which grow by adding carbon from the catalyst.
Yokomichi et al., xe2x80x9cEffects of High Magnetic Field on the Morphology of Carbon Nanotubes and Selective Synthesis of Fullerenes,xe2x80x9d Applied Physics Letters vol. 74, pp. 1827(1999) discloses that magnetic fields up to 10T affect curvature of nanotubes and increase the ratio of C70 to C60.
T. Ando, xe2x80x9cSpin-Orbit Interaction in Carbon Nanotubes,xe2x80x9d Journal of the Physical Society of Japan vol. 69(6), pp. 1757(1999) discloses the use of effective mass Hamiltonian in the presence of a weak spin-orbit interaction in order to demonstrate a spin Zeeman energy due to magnetic field in both the circumference and axial directions.
M. F. Lin, xe2x80x9cMagnetic Properties of Chiral Carbon Toroids,xe2x80x9d Physica B vol. 269, pp. 43-48(1999) discloses the dependence of magnetic properties of chiral carbon toroids reflects the character of xcfx80 electronic states and discloses that at high magnetic fields the Zeeman effect strongly affects low energy electronic structures, causing drastic diamagnetic to paramagnetic transitions.
Grossman et al. xe2x80x9cTransition Metals and their Carbides and Nitrides: Trends in Electronic and Structural Properties,xe2x80x9d Physical Review B vol. 60, pp. 6343 (1999) discloses the increase in moduli of transition metals with increasing number of electrons for a given principle quantum number, discloses similar moduli for n=3,4 with increase for n=5 and discloses the similarity trends of moduli pure metal and the corresponding metal carbides.
B. W. Smith et al.,xe2x80x9cStructural Anisotropy of Magnetically Aligned Single Wall Carbon Nanotube Films,xe2x80x9d Applied Physics Letters vol. 77(5) pp. 663(2000) discloses the nanotubular alignment occurs using a 25T strong magnetic field; the alignment is such that the axis orients parallel with the external field; and the nanotubular annealing in the strong magnetic field at high temperature.
Van der Wal et al., xe2x80x9cDiffusion Flame Synthesis of Single Walled Carbon Nanotubes,xe2x80x9d Chemical Physics Letters vol.323, pp. 217-223(2000) discloses the importance of C2 species in the molecular path toward soot formation.
Andriotis et al. xe2x80x9cCatalytic Action of Ni Atoms in the Formation of Carbon Nanotubes: A Molecular Dynamics Study,xe2x80x9d Physical Review Letters vol. 85, pp.3193-3196 (2000) discloses the theoretically predicted ability of Ni atoms to stabilize defects along a nanotube backbone until carbon atoms displace the Ni to eliminate the defect. Ni mobility within and without the nanotube and the curvature effects on Ni binding are disclosed.
On the basis of these references, the understanding of the phenomena of carbon formation from hydrocarbon decomposition on metal surfaces has enormously evolved. During the last 25 years, these investigations have determined processing conditions leading to differing phenomena such as graphitic fiber formation, graphitic nanotubular formation, fullerene formation and graphitic encapsulation of metal particle. This progress has resulted in a larger scale selective production of carbon nanotubes with the exclusion of these undesired carbon products. However, currently the nanotubular production rate is not practical for commercialization. Furthermore, the detail mechanism is still unknown. The deficits in the older art result from attempts to understand this complex process by models that are too simple. Previous efforts to increase rates have failed, because new complicating factors are not accounted for by extending the older model to this more complicated system.
On the basis of these previous investigations, the mechanism has been interpreted by the older art consisting of a simpler chemical catalytic model involving: an adsorption of hydrocarbon to a catalytic facet of the catalyst; the decomposition of the hydrocarbon on this facet; the desorption of H2 from this decomposing facet; the diffusion of carbon into and through the catalyst; and finally the precipitation of graphite at some other facet of the crystal. The older art does not recognize the extreme sensitivity of these processes to temperature. The diffusion of carbon within the catalyst has been determined as the rate-limiting step. The carbon diffusion has been attributed to concentration and temperature gradients between the decomposing and graphitizing facets of the catalyst. The diffusion bottleneck to faster nanotube formation results in carbon supersaturation with subsequent carbiding and poisoning of the catalyst. Large temperature fluctuations cause supersaturation and poisoning under high carbon-concentration conditions. Currently during CCVD extended growth is interrupted by sporadic temperature fluctuation and poisoning, shortening growth time and productivity. Hence, efforts to increase reaction and formation rates by classical strategies of increasing the temperature and the concentration gradients have failed. Catalytic deactivation occurs as a result of the supersaturation caused by these older strategies. The supersaturation causes global precipitation with the formation of a carbonaceous overlayer at the catalytic facet. Although promoters such as S, Cu and H2 reduce the poisoning they still do not eliminate the poisoning. In addition to catalytic poisoning, these mass production efforts of the older art have failed due to competing formations of multiwalled nanotubes and amorphous carbon deposits, whereby the system alleviates the supersaturation by nucleating internal tubes and unhybridized amorphous carbon. These limitations of the existing nanotube preparation technology have limited the process to the formation of large quantities of shorter nanotubes with efforts to sinter the tubes to form longer tubes.
I have discovered a new CVD technique to reduce the poisoning, thereby allowing bulky formation of nanotubes. This discovery reduces carbon concentration in the catalyst and increases the carbon diffusion through the catalyst by slowing the carbon production, lowering the activation energy for carbon diffusion, and increasing the driving force for carbon diffusion rather than introducing supersaturating temperature and concentration conditions. This new approach therefore lowers interior and surface carbon concentrations and increases carbon flux, thereby reducing the tendency for supersaturation and poisoning. Furthermore, this new method binds the growing nanotube to the metal nanoparticle for a more extended growth period. This new art of CCVD is based upon my new mechanism. My new mechanism accounts for complicating factors, not included in the traditional upscaling strategies. My new mechanism modifies the previous model by accounting for magnetic effects of the catalyst.
The most efficient catalysts for nanotube formation are all ferromagnetic: Fe, Ni and Co. Paramagnetism is the attraction of a substance to a magnetic field due to unpaired electrons of atoms in the material. However, the atoms of paramagnetic substances do not self-orient. Diamagnetism is the repulsion of a substance by a magnetic field due to the pairing of electrons in the atoms making up the material. Ferromagnetism is a special case of paramagnetism where in the magnetic moment and exchange between atoms lead to the spontaneous orientation in the absence of an external magnetic field. The magnetic forces are usually weaker than the electric forces, so these magnetic effects have been ignored. This reasoning accounts for the exclusion of the magnetic factor by the older art. However, I attribute the unique catalytic abilities of Co, Fe and Ni for nanotubular formation to the large magnetic moments of these metals as nanoparticles. Such large moments produce an intrinsic magnetic field which decelerates decomposition of the hydrocarbon, lowers surface and interior carbon concentration below saturation limit, forces the H2 product away from the surface after the decomposition, pulls the resulting carbon atoms on the surface into the interior of the catalyst particle, increases and directs carbon mobility by lowering activation energy for diffusion, promotes graphitic crystallization at some lateral facet of the catalyst particle, bends the graphene sheet nucleating the nanotube, inhibits multi walled formation relative to single wall nanotubular growth and binds the growing tube to the nanoparticle. The magnetic field also restricts the graphitization to only one facet of the particle. Because of these serious effects of the magnetic field on the mechanism, the magnetic factor cannot be neglected. The new art is based upon a novel magnetic model and incorporates enhancing the phenomena by externally driving the phenomena by applying an external, strong magnetic field.
Based on my new mechanism, the magnetic field is crucial for nanotube formation and growth. Therefore, any effort to substantially increase growth rate must modify the magnetic field. My discovery pertains to using a strong external magnetic field to enhance the individual processes occurring during the formation and growth. Because the carbon diffusion is the rate-limiting step and the magnetic properties of the catalyst accelerate diffusion, this invention focuses upon using an external magnetic field to lower interior carbon concentration and lower the activation energy and increase the directional driving force for carbon mobility beyond the reductions caused by the intrinsic magnetic field. This technique is called MAGNETO CATALYTIC CHEMICAL VAPOR DEPOSITION.